High pressure discharge lamps



Oct. 27, 1970 TADATQSHI'jHIGASl-ll ET AL 3,536,947

HIGH PRESSURE DISCHARGE LAMPS Filed March 20, 1968 2 Sheets-Sheet 1 Relative spectral intensity N BY M W Oct. 27, 1970 TADATOSHITHIGASHI ETAL 3,536,947

' HIGH PRESSURE DISCHARGE LAMPS Filed March 20, 1968 2 Sheets-Sheet 2 FIG. 3

Relative spectral intensity E 4? Wave1eng1h(mi11imicron) 10 FIG. 4

Relofive spectral intensify a.

O 400 soo soo 70o WavelengtMmillimicron) TM BY M M4 INVENTORS United States Patent 3,536,947 HIGH PRESSURE DISCHARGE LAMPS Tadatoshi Higashi and Satoshi Nagano, Kawasaki-shi, and Leo Mori, Tokyo, Japan, assignors to Tokyo Shibaura Electric Co., Ltd., Kawasaki-shi, Japan, a corporation of Japan Filed Mar. 20, 1968, Ser. No. 714,521 Claims priority, application Japan, Mar. 23, 1967, 42/17,633 Int. Cl. H01j 61/18 US. Cl. 313-229 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates ot a high pressure discharge lamp and more particularly to a discharge lamp having a high efficiency and an excellent colour rendering property because of a continuous spectrum over the entire range of visible rays and, which is suitable for use as a source of illumination for general purposes.

An ordinary high pressure discharge lamp comprises an evacuated envelope and a luminous tube made of quartz glass and containing an ionisable rare gas such as argon under a pressure of several centimeters of Hg, several milligrams of mercury per cubic centimeter of the inside volume of the luminous tube and halides such as bromides or iodides of sodium, lithium, thallium, indium, scandium, thorium and the like. The luminous tube is contained in the evacuated envelope. Discharge lamps are already known which utilise as said halides, (A) halides of thallium, or halides of sodium or mixtures thereof, (B) a mixture of iodides of thallium, sodium and indium, (if required lithium may be added to this mixture) and (C) a mixture of iodides of sodium and scandium.

In the case of (A), a discharge lamp of the rated input of 400 watts and employing halides of thallium manifests an efficiency of 80 lumens per watt and emanates light containing a large green component. On the other hand, when a halide of sodium is used an efiiciency of 65 lumens per watt is obtainable and the emanated light contains a strong orange component. Colour rendering property of these lamps are poor. Colour rendering property can be improved by suitable choice of the proportions of mixing thallium and sodium but not to a sufiicient extent for general lighting purpose. In the cases of (B) and (C), with a lamp of 400 watts, it is possible to produce white light at efliciencies of more than 75 lumens per watt. However, since these discharge lamps utilise iodides of sodium, sodium tends to be lost through the wall of the luminous tubes when they are operated over a long period of time with the result that iodine becomes excessive causing voltage fluctuation across discharge lamps. It is also true when iodides of sodium are used as the halides thereof in the case of (A). In the cases of (B) and (C), although the efliciency of discharge lamps can be improved, the efficiency of lamps operating at an input of 400 watts does never exceed 85 lumens per watt thus requiring further improvements for practical use.

It is therefore the principal object of this invention to provide a high pressure discharge lamp which can emanate a continuous spectrum over the entire range of visible light, has an improved colour rendering property and high efliciency, and can operate stably over a long operating period.

According to this invention this object can be accomplished by sealing in a luminous tube a rare gas, mercury, at least one element selected from particular rare earth elements, thallium and a halogen in a predetermined range.

More particularly in accordance with this invention there is provided a discharge lamp comprising a luminous tube including discharge electrodes, said luminous tube containing an ionisable starting rare gas, from 0.5 to 10 milligrams per cubic centimeter of mercury, 1.2 10 to 1.2 10 gram-atoms per cubic centimeter of at least one rare earth element selected from the group consisting of thulium, erbium and holmium 1.0 10 to 4 10- gram-atoms per cubic centimeter of thallium and a halogen of the quantity of more than 4.6 l0-' gram-atoms per cubic centimeter but less than the total sum of one-third of the chemical equivalent of said mercury, the chemical equivalents of said rare earth elements and the chemical equivalent of thallium.

Further objects and advantages of the present invention will become apparent and this invention will be better understood from the following description, reference being made to the accompanying drawings, in which:

FIG. 1 shows an elevation of an electric discharge lamp embodying this invention; and

FIGS. 2 to 4 are graphs illustrating the relative spectral energy distribution characteristics of light rays emitted from electric discharge lamps embodying this invention;

Referring now to the accompanying drawings, in FIG. 1 is shown an electric discharge lamp comprising an evacuated envelope 1 made of transparent glass and provided with a base 2 at one end thereof.

A supporting frame 3 made of a metal wire is disposed Within the envelope to support a luminous sealed tube 4 which is made of a transparent fused quartz glass tube with its opposite ends pressed.

Within the glass tube 4 are disposed a main electrode 5 at one end, and another main electrode 6 and an auxiliary electrode 7 at the other end, which are respectively supported by ribbon shaped conductors =8, 9 and 10 sealed in said pressed portions. The main electrode 5 is electrically connected to one terminal of the base via the supporting frame 3 and a lead-in conductor 11 sealed in a stem 12, while the other main electrode 6- is connected to the other terminal of the base via lead-in conductor 13 similarly sealed in the stem 12. The auxiliary electrode 7 is connected to the supporting frame 3 through a conductor 14 and a starting resistor 15. It should be understood that the auxiliary electrode 7 may be omitted when desired. In the sealed tube 4 are sealed ionisable starting rare gas such as argon at a pressure of 10 to 40 millimeters of Hg, mercury of the quantity of 0.5 to 10 milligrams, thallium of the quantity of from 1.0 10* to 4 10* gram-atoms, from l.2 10"' to 1.2 10 gram-atoms of at least one rare earth element, selected from the group consisting of thulium, erbium and holmium and a halogen, for example iodine, of the quantity of more than 4.6 X 10- gram-atoms but less than the sum of one third of the chemical equivalent of said mercury, the chemical equivalent of the rare earth element and the chemical equivalent of thallium, all of said gram molecules rbeing per cubic centimeter of the inner volume of the sealed tube.

When impressed with a starting voltage from a conventional starting device, the electric discharge lamp constructed as above described will initiate a glow discharge between the main electrode 6 and the auxiliary electrode 7 followed by an arc discharge between the main electrodes and 6. The heat generated by this discharge will heat up the sealed tube 4, thus increasing the vapour pressure of mercury, halides of and rare earth elements which are sealed therein. The vapourised halides of rare earth elements emit an intense continuous spectrum in the visible range, while at the same time thallium and rare earth elements emit continuously distributed inherent lines in the spectrum.

In this way, thallium, rare earth elements and their halides utilised according to this invention will act to complement the light in the wavelength region of the visible spectrum which is not afforded by mercury with the. result that the spectral energy distribution is made more uniform and the colour rendering property is improved. The colour rendering property can be further enhanced by providing on the inner wall of the envelope 1 a phosphorescent layer which emits wavelengths in the red chiefly by absorbing ultraviolet.

The reason that the quantities of mercury, rare earth elements, thallium and halogens sealed in the luminous tube of the discharge lamp embodying this invention are selected to be in the range mentioned above is as follows: More particularly, with mercury of the quantity outside the range of from 0.5 to milligrams per cubic centimeter of the inner volume of the luminous tube, the desired luminescence or electrical characteristics cannot be expected, whereas with one, two or more of the rare earth elements selected from the group consisting of thulium, erbium and holmium of the quantity of less than 1.2 X 10- gram molecules per cubic centimeter of the inner volume of the luminous tube the advantage of sealing them in is not sufficient. When the quantity of these elements exceeds 1.2 10 gram-atoms, the elements tend to deposit on the wall of the tube thus decreasing the light flux. Said materials may be sealed in singly or in the form of halides. The type of the rare earth elements is limited to the above mentioned three types and other elements can not accomplish the object of this invention or can not provide discharge lamps of high luminescent efiiciencies. Also the quantity of the thallium sealed in the luminous tube should be specified in order to manifest excellent characteristics by the cumulative function with the rare earth elements such as thulium. In accordance with the present invention. the quantity of thallium used is selected to be less than 4 l0- gram-atoms per cubic centimeter in order to avoid such problems as the instability of the arc and the decrease in the short wave components ranging from bluish purple to blu due to reabsorption and to be more than 1.-0 10 gram-atoms per cubic centimeter in order to provide high efficiency discharge lamps by emanating the desired light flux. This quantity is much smaller than that of a conventional discharge lamp containing more than 6 10 gram-atoms per cubic centimeter of thallium iodide in addition to the requisite quantities of mercury and argon. Further the quantity of halogen to be sealed in should be at least 4.6x l0" gram-atoms per cubic centimeter in order to provide desired luminescence by the cumulative action with metals, and should not exceed the sum of the chemical equivalents of said rare earth elements, the chemical equivalent of thallium, and one third of the chemical equivalent of mercury, otherwise the discharge becomes unstable and excellent operating characteristics cannot be expected. Moreover excessive halogen may corrode the electrodes.

The discharge lamp of this invention constructed as above described readily emits white light at high efiiciencies of from 80 lumens to 90 lumens per watt with an input of 400 watts for example, when applied with a starting voltage of from 300 to 400 volts from a well known lighting device. As the white light is distributed as a continuous spectrum over the entire visible range the discharge lamp has high practical value as the light source for general purpose. With the halide additive discharge lamp, immediately after striking a discharge between discharge electrodes by the application of the starting voltage essentially the mercury spectrum lines are emanated which results in the increase in the wall temperature of the luminous tube. This temperature increase causes vapourisation of the thallium halide, for example, thallium iodide. Then the resonance line of thallium of a wavelength of 5,350 angstrom becomes predominent. When the wall temperature further increases to the steady state of 680 to 800 C. (at which time the tube wall load is equal to at least 12 watts per cubic centimeter) a continuous spectrum is emitted by the halides of said rare earth elements with the result that the resonance line of thallium becomes relatively weak thus emanating white light at high efficiencies as a whole. Luminescence of white light at these high efiiciencies is diiferent dependent upon thetype of the rare earth elements used. The efficiency is the highest for thulium and can amount to more than 90 lumens per watt at an input of 400 watts. The light is bluish white to white giving cool feeling. In the case of holmium or erbium the efficiency amounts to lumens per watt at the same input of 4 00 watts and yet the colour rendering property of the lamp is better than that using thulium. As a result, when a suitable combination of these rare earth elements are used it is possible to provide discharge lamps of any one of different efficiencies and diflerent colour rendering properties.

As above described the discharge lamp emits white light having a continuous spectrum distributed over the entire visible range at high efficiencies of 80 to lumens per watt and which is suitable as a light source for general purposes. Thus, it will be clear that the discharge lamp has higher practical values when compared with a discharge lamp containing rare earth elements, mercury and argon but not thulium and can emit at most 65 lumens per watt at an input of 400 watts since such a lamp emanates plenty of relatively short wavelengths from blue to bluish purple and with a discharge lamp containing thallium iodide and mercury but not containing rare earth elements and has relatively high efficiencies of 75 to 80 lumens per watt and yet unsuitable for ordinary illumination owing to its green colour.

The following specific examples are given by way of illustration, and are not to be construed as limiting in any way the scope and spirit of the invention.

EXAMPLE 1 A discharge lamp including a luminous tube made of a sealed off fused quartz tube having an inside diameter of 16 millimeters, an arc length of 50 millimeters, and an inside volume of 12- cubic centimeters was prepared and in which were sealed 30 milligrams of mercury, 1.5 l0- gram-atoms of thallium iodide, 2 10 gramatoms of thulium iodide and argon gas at a pressure of 20 millimeters of Hg at room temperature. When energised with an input of 400 watts the discharge lamp emanated white light at a colour temperature of 6,000 K. (absolute temperature) with a total light flux of 36,400 lumens. Immediately after initiating discharge the lamp emanated essentially the spectrum lines of mercury, but with the increase of the tube wall temperature the resonant line of thallium having a wavelength of 5,353 angstrom became predominant, and when the tube wall temperature further increased to the steady state a continuous spectrum due to thulium iodide was emitted with the result that the resonance line of thallium became relatively weak. FIG. 2 shows the spectral energy distribution at this state.

In this example, when thulium iodide was substituted by 2x10 gram-atoms of holmium iodide the discharge lamp emanated white light of good quality having a total light flux of 34,000 lumens when energised with an in put of 400 watts. FIG. 3 shows the spectral energy distribution of this modified lamp. In this modified example even when erbium was substituted for holmium iodide substantially the same advantage was noted.

EXAMPLE 2 A discharge lamp including a luminous tube made of a sealed off fused quartz tube having an inside diameter of 16 millimeters, an arc length of 50 millimeters and an inside volume of 12 cubic centimeters and provided with a fluorescent layer including a rare earth element on the inner wall of the outer bulb was prepared and in which were sealed 30 milligrams of mercury, x10 gram-atoms of thallium iodide, 140x10 gram-atoms of thulium iodide, 1.0 1O- gram-atoms of holmium iodide and argon gas at a pressure of 20* millimeters at room temperature. When energised with an input of 400 watts the discharge lamp emanated a total light flux of 35,000 lumens with an excellent colour rendering property.

In the above discharge lamp thulium iodide was substituted for a mixture of holmium iodide and thulium iodide and the spectral distribution of the light emanated therefrom when energised With an input of 400 watts was measured and the result is shown in FIG. 4. The total light flux was 36,200 lumens per watt.

Thus it was found that the provision of the fluorescent layer had an effect of improving the colour rendering property without any appreciable decrease in the total light flux.

While the invention has been described in connection with some preferred embodiments thereof, the invention is not limited thereto and includes any modifications and alternations which fall within the true spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A high pressure discharge lamp comprising a luminous tube including discharge electrodes, said luminous tube containing starting rare gas, from 0.5 to 10 milligrams per cubic centimeter of mercury, 1.2 10-' to 1 .2 10- gram-atoms per cubic centimeter of at least one rare earth element selected from the group consisting of thulium, erbium and holmium, 1.0 l 0-" to 4X10- gram-atoms per cubic centimeter of thallium and a halogen of the quantity of more than 4.6x 10 gram-atoms per cubic centimeter but less than the total sum of one third of the chemical equivalent of said mercury, the chemical equivalent of said rare earth element and the chemical equivalent of thallium.

2. A high pressure discharge lamp comprising a luminous tube including an envelope with an inner wall, discharge electrodes, said luminous tube containing starting rare gas, from 0.5 to 10 milligrams per cubic centimeter of mercury, 1.2 10- to 1.2 10- gram-atoms per cubic centimeter of at least one rare earth element selected from the group consisting of thulium, erbium and holmium, 1. 0 10 to 4x1 0 gram-atoms per cubic centimeter of thallium and a halogen of the quantity of more than 46x10 gram-atoms per cubic centimeter but less than the total sum of one third of the chemical equivalent of said mercury, the chemical equivalent of said rare earth element and the chemical equivalent of thallium, said envelope inner wall being coated with a phosphor material which emits Wavelengths in the red chiefly by absorbing ultraviolet.

References Cited UNITED STATES PATENTS RAYMOND F. HOSSFELD, Primary Examiner 

